(a) Field of the Invention
This invention relates to the developing of an enhanced anomalous resistivity pattern in a geological sedimentary subsurface and more particularly, but not by way of limitation, to a method and apparatus for providing enhanced resistivity subsurface patterns associated with oil and gas traps.
(b) Discussion of Prior Art
Heretofore, there have been a number of different U.S. Patents which describe the use of electrical current passed through the earth's surface for measuring an electric field and magnetic field in the subsurface along with measuring resistivity of rock formations. For example in U.S. Pat. No. 2,730,673 to Jakosky, an older electromagnetic method of subsurface surveying is described using an electric current passed through the earth's surface for measuring an induced magnetic field created by the current flow. In U.S. Pat. No. 2,796,235 to Quigley a process of geophysical prospecting particularly in areas of high electrical resistivity such as gilsonite dikes is disclosed.
U.S. Pat. Nos. 4,041,372 to Miller et al., 4,446,434 to Sternberg et al., 4,467,283 to Owen et al. disclosed geophysical exploration systems using a plurality of spaced apart probes with addressable switching units for providing calculable electromagnetic coupling for indirect detection of hydrocarbon reservoirs. The resistivity of the rock is measured and compared with anomaly models using a method of comparing measured phase angle differences between stored current characteristics and stored voltage characteristics. The methods described in these patents have limited depth of penetration in the subsurface.
U.S. Pat. No. 4,835,473 to Bostick describes a method of determining resistivity using a dc current source wherein variations in the earth's magnetic field are measured in two, non-parallel directions. U.S. Pat. No. 4,837,514 to Spies describes a method of reducing noise during electromagnetic exploration and U.S. Pat. No. 4,875,015 to Ward discusses an improved method related to borehole resistivity.
None of the above mentioned patents disclose or teach a method and apparatus for specifically looking for hydrocarbon traps wherein rock formation resistivity changes take place above a potential oil and gas reservoir and further used to penetrate an entire sedimentary section down to basement rock and below to depths up to 20,000 feet, and deeper under certain conditions. Also, none of these patents discuss using a method coupled with removing a horizontally layered earth model in order to enhance the anomalous resistivity pattern caused by the presence of an oil and gas accumulation.
Because the subject invention has a large depth of penetration with a high degree of resolution in measuring rock resistivity, it is ideally suited for oil and gas exploration.